1. Field of the Invention
The present invention relates to a rubber composition which can be applied to various products such as tires, vibration isolation rubbers and belts and, more particularly, to a rubber composition which exhibits an excellent gripping property and excellent resistance to fracture and is advantageously applied to treads of pneumatic tires.
2. Description of the Related Arts
As higher safety in automobile driving is attracting attention in recent years, a more excellent gripping property in both good weather and bad weather is required for automobile tires. In particular, improvement in the gripping property is important for decreasing the braking distance of an automobile. To satisfy this requirement, it is desired that ability of a rubber composition used for a tire tread to grip the road surface be improved to a still higher level.
Various developments have been made to obtain a rubber composition having an excellent gripping property. As a typical example of such development, a highly filled rubber composition containing large amounts of oil and filler which has an excellent gripping property in good weather (dry gripping property) has been provided. A highly filled rubber composition containing large amounts of oil and filler has a large tan xcex4 value in the temperature-frequency region relating to gripping property so that kinetic energy can be converted into thermal energy effectively, and an excellent gripping property is exhibited. In other words, to enhance dry gripping property of a rubber composition, it is important that hysteresis loss property, which can be represented by tan xcex4, in the temperature range from room temperature to about 100xc2x0 C. is increased. When such a highly filled rubber composition is used, however, a drawback arises in that sufficient entanglements among polymer molecules may not surely be formed since an oil having a low molecular weight is used, and fracture resistance may decrease although an excellent loss property can be surely exhibited.
To prevent a decrease in fracture resistance, it is effective that a low molecular weight polymer having a higher molecular weight than that of an oil is used as a softener. Some problems arise, however, in the use of such a low molecular weight polymer.
When a low molecular weight diene polymer which is heretofore attempted to be used, i.e., a liquid styrene-butadiene copolymer (liquid SBR) or a liquid polybutadiene (liquid BR), is used, a crosslinking reaction takes place in the polymer during vulcanization of the rubber composition since the diene polymer has double bonds in the molecule. Therefore, the obtained rubber product does not always exhibit a satisfactory modulus. Moreover, the molecular weight is limited in order to obtain a sufficient loss property.
To overcome the above problems, the use of a low molecular weight saturated polymer has been attempted. Liquid polybutene is a typical example of the low molecular weight saturated liquid polymer used for rubber compositions. When a saturated polymer is used, a drawback arises in that blooming takes place on the rubber composition due to insufficient compatibility with the diene polymer compounded as a rubber component.
As another method for improving the gripping property, it is known that rubber component having a high glass transition temperature can be used for the rubber composition for a tire tread. When a rubber having a high glass transition temperature is used, however, the temperature range in the use of the tire should be limited since the rubber tends to become rigid in low temperature range. Therefore, it is difficult that a rubber having a high glass transition temperature is used for such a rubber composition.
From the standpoint of safety, it is required that fracture resistance and gripping property of the tread rubber be further improved.
Under above circumstances, the present invention has an object of providing a rubber composition which exhibits an excellent gripping property, an excellent fracture resistance and an excellent abrasion resistance simultaneously by using a novel formulation.
As the result of intensive studies by the present inventors to achieve the above object with particular attention paid to improvement in the compatibility between a saturated polymer and a diene rubber, it was found that an excellent fracture resistance, an excellent abrasion resistance and an excellent dry gripping property can be simultaneously exhibited by using a copolymer of a monoolefin and an aromatic vinyl compound, which is not heretofore used in formulations of a rubber composition, in combination with a diene polymer conventionally used as a rubber component. The present invention has been completed based on the knowledge.
The present invention provides a rubber composition which is obtained by compounding 100 parts by weight of a rubber component and 3 to 250 parts by weight of an olefin-aromatic vinyl compound copolymer.